Leadless cardiac pacemaker system for usage in combination with an implantable cardioverter-defibrillator

ABSTRACT

A cardiac pacing system comprising one or more leadless cardiac pacemakers configured for implantation in electrical contact with a cardiac chamber and configured to perform cardiac pacing functions in combination with a co-implanted implantable cardioverter-defibrillator (ICD). The leadless cardiac pacemaker comprises at least two leadless electrodes configured for delivering cardiac pacing pulses, sensing evoked and/or natural cardiac electrical signals, and bidirectionally communicating with the co-implanted ICD.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.15/208,194, filed Jul. 12, 2016, which is a continuation of U.S. patentapplication Ser. No. 14/885,853, filed Oct. 16, 2015, now U.S. Pat. No.9,409,033, which is a divisional of U.S. patent application Ser. No.14/318,201, filed Jun. 27, 2014, now U.S. Pat. No. 9,192,774, which is adivisional of U.S. patent application Ser. No. 13/866,803, filed Apr.19, 2013, now U.S. Pat. No. 8,798,745, which is a divisional of U.S.patent application Ser. No. 11/549,599, filed Oct. 13, 2006, now U.S.Pat. No. 8,457,742, which application claims the benefit of priority toand incorporates herein by reference in its entirety for all purposes,U.S. Provisional Patent Application Nos.: 60/726,706 entitled “LEADLESSCARDIAC PACEMAKER WITH CONDUCTED COMMUNICATION,” filed Oct. 14, 2005;60/761,531 entitled “LEADLESS CARDIAC PACEMAKER DELIVERY SYSTEM,” filedJan. 24, 2006; 60/729,671 entitled “LEADLESS CARDIAC PACEMAKER TRIGGEREDBY CONDUCTED COMMUNICATION,” filed Oct. 24, 2005; 60/737,296 entitled“SYSTEM OF LEADLESS CARDIAC PACEMAKERS WITH CONDUCTED COMMUNICATION,”filed Nov. 16, 2005; 60/739,901 entitled “LEADLESS CARDIAC PACEMAKERSWITH CONDUCTED COMMUNICATION FOR USE WITH AN IMPLANTABLECARDIOVERTER-DEFIBRILLATOR,” filed Nov. 26, 2005; 60/749,017 entitled“LEADLESS CARDIAC PACEMAKER WITH CONDUCTED COMMUNICATION AND RATERESPONSIVE PACING,” filed Dec. 10, 2005; and 60/761,740 entitled“PROGRAMMER FOR A SYSTEM OF LEADLESS CARDIAC PACEMAKERS WITH CONDUCTEDCOMMUNICATION,” filed Jan. 24, 2006; all by Peter M. Jacobson.

BACKGROUND

Cardiac pacing electrically stimulates the heart when the heart'snatural pacemaker and/or conduction system fails to provide synchronizedatrial and ventricular contractions at appropriate rates and intervalsfor a patient's needs. Such bradycardia pacing provides relief fromsymptoms and even life support for hundreds of thousands of patients.Cardiac pacing may also give electrical overdrive stimulation intendedto suppress or convert tachyarrhythmias, again supplying relief fromsymptoms and preventing or terminating arrhythmias that could lead tosudden cardiac death.

Cardiac pacing is usually performed by a pulse generator implantedsubcutaneously or sub-muscularly in or near a patient's pectoral region.Implantable cardioverter-defibrillator (ICD) pulse generators usuallyinclude cardiac pacing functions, both for bradycardia support and foroverdrive stimulation. The generator usually connects to the proximalend of one or more implanted leads, the distal end of which contains oneor more electrodes for positioning adjacent to the inside or outsidewall of a cardiac chamber. The leads have an insulated electricalconductor or conductors for connecting the pulse generator to electrodesin the heart. Such electrode leads typically have lengths of 50 to 70centimeters.

A conventional pulse generator may be connected to more than oneelectrode-lead. For example, atrio-ventricular pacing, also commonlycalled dual-chamber pacing, involves a single pulse generator connectedto one electrode-lead usually placed in the right atrium and a secondelectrode-lead usually placed in the right ventricle. Such a system canelectrically sense heartbeat signals and deliver pacing pulsesseparately in each chamber. In typical use, the dual-chamber pacingsystem paces the atrium if no atrial heartbeat is sensed since apredetermined time, and then paces the ventricle if no ventricularheartbeat is sensed within a predetermined time after the natural orpaced atrial beat. Such pulse generators can also alter the timing ofatrial and ventricular pacing pulses when sensing a ventricular beatthat is not preceded by an atrial beat within a predetermined time; thatis, a ventricular ectopic beat or premature ventricular contraction.Consequently, dual-chamber pacing involves pacing and sensing in anatrium and a ventricle, and internal communication element so that anevent in either chamber can affect timing of pacing pulses in the otherchamber.

Recently, left-ventricular cardiac pacing has been practiced toameliorate heart failure; a practice termed cardiac resynchronizationtherapy (CRT). CRT has been practiced with electrode-leads and a pulsegenerator, either an implantable cardioverter-defibrillator (CRT-D) oran otherwise conventional pacemaker (CRT-P). The left-ventricular pacingconventionally uses an electrode in contact with cardiac muscle in thatchamber. The corresponding electrode-lead is usually placedendocardially in a transvenous manner through the coronary sinus vein,or epicardially. Left-ventricular pacing is usually practiced togetherwith right-atrial and right-ventricular pacing with a single implantedpulse generator connected to three electrode-leads. CRT pulse generatorscan independently vary the time between an atrial event andright-ventricular pacing, and the time between an atrial event andleft-ventricular pacing, so that the left ventricular pacing pulse canprecede, follow, or occur at the same time as the right-ventricularpacing pulse. Similarly to dual-chamber pacing, systems withleft-ventricular pacing also change atrial and ventricular pacing timingin response to premature ventricular contractions. Consequently, CRT-Dinvolves pacing in an atrium and in two ventricles, sensing in theatrium and at least one ventricle, and an internal communication elementso that an event in the atrium can affect timing of pacing pulses ineach ventricle, and an internal communication element so that an eventin at least one ventricle can affect timing of pacing pulses in theatrium and the other ventricle.

Pulse generator parameters are usually interrogated and modified by aprogramming device outside the body, via a loosely-coupled transformerwith one inductance within the body and another outside, or viaelectromagnetic radiation with one antenna within the body and anotheroutside.

Although more than one hundred thousand ICD and CRT-D systems areimplanted annually, several problems are known.

A conventional pulse generator has an interface for connection to anddisconnection from the electrode leads that carry signals to and fromthe heart. Usually at least one male connector molding has at least oneadditional terminal pin not required for defibrillation functions at theproximal end of the electrode lead. The at least one male connectormates with at least one corresponding female connector molding andterminal block within the connector molding at the pulse generator.Usually a setscrew is threaded in at least one terminal block perelectrode lead to secure the connection electrically and mechanically.One or more O-rings usually are also supplied to help maintainelectrical isolation between the connector moldings. A setscrew cap orslotted cover is typically included to provide electrical insulation ofthe setscrew. The complex connection between connectors and leadsprovides multiple opportunities for malfunction.

For example, failure to introduce the lead pin completely into theterminal block can prevent proper connection between the generator andelectrode.

Failure to insert a screwdriver correctly through the setscrew slot,causing damage to the slot and subsequent insulation failure.

Failure to engage the screwdriver correctly in the setscrew can causedamage to the setscrew and preventing proper connection.

Failure to tighten the setscrew adequately also can prevent properconnection between the generator and electrode, however over-tighteningof the setscrew can cause damage to the setscrew, terminal block, orlead pin, and prevent disconnection if necessary for maintenance.

Fluid leakage between the lead and generator connector moldings, or atthe setscrew cover, can prevent proper electrical isolation.

Insulation or conductor breakage at a mechanical stress concentrationpoint where the lead leaves the generator can also cause failure.

Inadvertent mechanical damage to the attachment of the connector moldingto the generator can result in leakage or even detachment of themolding.

Inadvertent mechanical damage to the attachment of the connector moldingto the lead body, or of the terminal pin to the lead conductor, canresult in leakage, an open-circuit condition, or even detachment of theterminal pin and/or molding.

The lead body can be cut inadvertently during surgery by a tool, or cutafter surgery by repeated stress on a ligature used to hold the leadbody in position. Repeated movement for hundreds of millions of cardiaccycles can cause lead conductor breakage or insulation damage anywherealong the lead body.

Although leads are available commercially in various lengths, in someconditions excess lead length in a patient exists and is to be managed.Usually the excess lead is coiled near the pulse generator. Repeatedabrasion between the lead body and the generator due to lead coiling canresult in insulation damage to the lead.

Friction of the lead against the clavicle and the first rib, known assubclavian crush, can result in damage to the lead.

In dual-chamber pacing in an ICD, and in CRT-D, multiple leads areimplanted in the same patient and sometimes in the same vessel. Abrasionbetween these leads for hundreds of millions of cardiac cycles can causeinsulation breakdown or even conductor failure.

Subcutaneous ICDs that do not use endocardial, transvenous or epicardiallead wires, can deliver defibrillation using subcutaneous electrodes.However, pacing the heart from subcutaneous electrodes results indiaphragmatic stimulation which is uncomfortable to the patient if usedin long-term therapy. Therefore pacing therapies such as bradycardiapacing therapy, anti-tachycardia therapy, atrial overdrive pacing forthe prevention of arrhythmias, dual chamber pacing for atrio-ventricularsynchronization and CRT therapies are inappropriate.

SUMMARY

According to an embodiment of a cardiac pacing system, one or moreleadless cardiac pacemakers are configured for implantation inelectrical contact with a cardiac chamber and configured for performingcardiac pacing functions in combination with a co-implanted implantablecardioverter-defibrillator (ICD). The leadless cardiac pacemakercomprises at least two leadless electrodes configured for deliveringcardiac pacing pulses, sensing evoked and/or natural cardiac electricalsignals, and bidirectionally communicating with the co-implanted ICD.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention relating to both structure and method ofoperation may best be understood by referring to the followingdescription and accompanying drawings, in which similar referencecharacters denote similar elements throughout the several views:

FIG. 1A is a pictorial diagram showing an embodiment of a cardiac pacingsystem including one or more leadless cardiac pacemakers with conductedcommunication for performing cardiac pacing in conjunction with animplantable cardioverter-defibrillator (ICD);

FIG. 1B is a schematic block diagram showing interconnection ofoperating elements of an embodiment of a leadless cardiac pacemaker thatcan be used in cardiac pacing system including one or more leadlesscardiac pacemakers and an implantable cardioverter-defibrillator (ICD);

FIG. 2 is a pictorial diagram showing the physical location of someelements of an embodiment of a leadless biostimulator that can be usedas part of a multi-chamber cardiac pacing system;

FIG. 3 is a pictorial diagram that depicts the physical location of someelements in an alternative embodiment of a leadless biostimulator thatcan be used as part of a multi-chamber cardiac pacing system;

FIG. 4 is a time waveform graph illustrating a conventional pacingpulse;

FIG. 5 is a time waveform graph depicting a pacing pulse adapted forcommunication as implemented for an embodiment of the illustrativepacing system;

FIG. 6 is a time waveform graph showing a sample pulse waveform usingoff-time variation for communication;

FIG. 7 is a state-mechanical representation illustrating an embodimentof a technique for operation of an atrial leadless cardiac pacemaker ina multi-chamber cardiac pacing system;

FIG. 8 is a state-mechanical representation illustrating an embodimentof a technique for operation of a right-ventricular leadless cardiacpacemaker in a multi-chamber cardiac pacing system;

FIG. 9 is a state-mechanical representation illustrating an embodimentof a technique for operation of a left-ventricular leadless cardiacpacemaker in a multi-chamber cardiac pacing system;

FIGS. 10A and 10B are schematic flow charts that depict embodiments ofmethods for operating an atrial leadless cardiac pacemaker in a cardiacpacing system including an implantable cardioverter-defibrillator (ICD)and one or more leadless cardiac pacemakers;

FIGS. 11A and 11B are schematic flow charts that depict embodiments ofmethods for operating a right-ventricular leadless cardiac pacemaker ina cardiac pacing system including an implantablecardioverter-defibrillator (ICD) and one or more leadless cardiacpacemakers; and

FIGS. 12A and 12B are schematic flow charts that depict embodiments ofmethods for operating a left-ventricular leadless cardiac pacemaker in acardiac pacing system including an implantablecardioverter-defibrillator (ICD) and one or more leadless cardiacpacemakers.

DETAILED DESCRIPTION

In some embodiments of an illustrative cardiac pacing system, one ormore leadless cardiac pacemakers with low-power conducted communicationcan perform single-chamber pacing, dual-chamber pacing, CRT-D, or otherpacing, co-implanted with an ICD, enabling functionality extendingbeyond what is possible or appropriate for conventional subcutaneousICDs.

A system of leadless cardiac pacemakers enables pacing in conjunctionwith an implantable cardioverter-defibrillator (ICD) for usage insingle-chamber, dual-chamber, CRT-D, and other multi-chamber cardiacpacing schemes.

Various embodiments of a system of an implantablecardioverter-defibrillator (ICD) and one or more leadless cardiacpacemakers are described. The individual leadless cardiac pacemakers canbe substantially enclosed in a hermetic housing suitable for placementon or attachment to the inside or outside of a cardiac chamber. Thepacemaker can have at least two electrodes located within, on, or nearthe housing, for delivering pacing pulses to and sensing electricalactivity from the muscle of the cardiac chamber, and for bidirectionalcommunication with at least one other co-implanted leadless cardiacpacemaker and optionally with another device outside the body. Thehousing can contain a primary battery to provide power for pacing,sensing, and communication. The housing can also contain circuits forsensing cardiac activity from the electrodes, receiving information fromat least one other device via the electrodes, generating pacing pulsesfor delivery via the electrodes, transmitting information to at leastone other device via the electrodes, monitoring device health, andcontrolling these operations in a predetermined manner.

A cardiac pacing system includes cardiac pacing in conjunction with anICD and can supplement the functionality of ICDs with cardiac pacingfunctions, extending beyond functionality of conventional ICD-pacingarrangements.

The cardiac pacing system comprises a leadless cardiac pacemaker orpacemakers adapted to perform cardiac pacing functions with aco-implanted ICD, without a pacing electrode-lead separate from theleadless cardiac pacemaker, without a communication coil or antenna inthe leadless cardiac pacemaker, and without an additional requirement onbattery power in the leadless cardiac pacemaker for transmittedcommunication.

In some embodiments, a cardiac pacing system comprises an ICD with oneor more leadless pacemakers for implantation adjacent to the inside oroutside wall of a cardiac chamber, without the need for a connectionbetween the leadless pulse generator and an electrode lead that can beconnected or disconnected during implantation and repair procedures, andwithout the need for a lead body.

In some embodiments of a cardiac pacing system, communication betweenthe implanted leadless cardiac pacemaker or pacemakers and otherdevices, including any co-implanted leadless cardiac pacemakers, theco-implanted ICD, and optionally a device external to the body, usesconducted communication via the same electrodes used for pacing, withoutthe need for an antenna or telemetry coil.

Some embodiments and/or arrangements can implement communication betweenan implanted leadless cardiac pacemaker and other devices with powerrequirements similar to those for cardiac pacing, to enable optimizationof battery performance. For example, transmission from the leadlesscardiac pacemaker adds no power while reception adds a limited amount ofpower, such as about 25 microwatt.

A cardiac pacemaker or pacemakers are adapted for implantation in thehuman body. In a specific embodiment, one or more leadless cardiacpacemakers can be co-implanted with an implantablecardioverter-defibrillator (ICD). Each leadless cardiac pacemaker usestwo or more electrodes located within, on, or within two centimeters ofthe housing of the pacemaker, for pacing and sensing at the cardiacchamber, for bidirectional communication with the ICD, optionally withat least one other leadless cardiac pacemaker, and optionally with atleast one other device outside the body.

Referring to FIG. 1A, a pictorial diagram shows an embodiment of acardiac pacing system 100 including one or more leadless cardiacpacemakers 102 with conducted communication for performing cardiacpacing in conjunction with an implantable cardioverter-defibrillator(ICD) 106. The system 100 can implement for example single-chamberpacing, dual-chamber pacing, or three-chamber pacing for cardiacresynchronization therapy, without requiring pacing lead connections tothe defibrillator 106. The illustrative cardiac pacing system 100comprises at least one leadless cardiac pacemaker 102 configured forimplantation in electrical contact with a cardiac chamber 104 andconfigured to perform cardiac pacing functions in combination with aco-implanted implantable cardioverter-defibrillator (ICD) 106. One ormore of the leadless cardiac pacemakers 102 can comprise at least twoleadless electrodes 108 configured for delivering cardiac pacing pulses,sensing evoked and/or natural cardiac electrical signals, anduni-directionally or bi-directionally communicating with theco-implanted ICD 106.

The leadless cardiac pacemakers 102 can communicate with one anotherand/or communicate with a non-implanted programmer and/or the implantedICD 106 via the same electrodes 108 that are also used to deliver pacingpulses. Usage of the electrodes 108 for communication enables the one ormore leadless cardiac pacemakers 102 for antenna-less and telemetrycoil-less communication.

The leadless cardiac pacemakers 102 can be configured to communicatewith one another and to communicate with a non-implanted programmer 106via communication that has outgoing communication power requirementsessentially met by power consumed in cardiac pacing.

In some embodiments, the individual leadless cardiac pacemaker 102 cancomprise a hermetic housing 110 configured for placement on orattachment to the inside or outside of a cardiac chamber 104 and atleast two leadless electrodes 108 proximal to the housing 110 andconfigured for bidirectional communication with at least one otherdevice 106 within or outside the body. For example, FIG. 1B depicts asingle leadless cardiac pacemaker 102 and shows the pacemaker'sfunctional elements substantially enclosed in a hermetic housing 110.The pacemaker 102 has at least two electrodes 108 located within, on, ornear the housing 110, for delivering pacing pulses to and sensingelectrical activity from the muscle of the cardiac chamber, and forbidirectional communication with at least one other device within oroutside the body. Hermetic feedthroughs 130, 131 conduct electrodesignals through the housing 110. The housing 110 contains a primarybattery 114 to supply power for pacing, sensing, and communication. Thehousing 110 also contains circuits 132 for sensing cardiac activity fromthe electrodes 108, circuits 134 for receiving information from at leastone other device via the electrodes 108, and a pulse generator 116 forgenerating pacing pulses for delivery via the electrodes 108 and alsofor transmitting information to at least one other device via theelectrodes 108. The housing 110 can further contain circuits formonitoring device health, for example a battery current monitor 136 anda battery voltage monitor 138, and can contain circuits 112 forcontrolling operations in a predetermined manner.

The one or more leadless electrodes 108 can be configured to communicatebidirectionally among the multiple leadless cardiac pacemakers and/orthe implanted ICD to coordinate pacing pulse delivery using messagesthat identify an event at an individual pacemaker originating themessage and a pacemaker receiving the message react as directed by themessage depending on the origin of the message. A pacemaker orpacemakers that receive the message react as directed by the messagedepending on the message origin or location. In some embodiments orconditions, the two or more leadless electrodes 108 can be configured tocommunicate bidirectionally among the one or more leadless cardiacpacemakers 102 and/or the ICD 106 and transmit data including designatedcodes for events detected or created by an individual pacemaker.Individual pacemakers can be configured to issue a unique codecorresponding to an event type and a location of the sending pacemaker.

Information communicated on the incoming communication channel caninclude but is not limited to pacing rate, pulse duration, sensingthreshold, and other parameters commonly programmed externally inconventional pacemakers. Information communicated on the outgoingcommunication channel can include but is not limited to programmableparameter settings, pacing and sensing event counts, battery voltage,battery current, device health, and other information commonly displayedby external programmers used with conventional pacemakers. The outgoingcommunication channel can also echo information from the incomingchannel, to confirm correct programming.

In some embodiments, information encoded on the leadless cardiacpacemakers can be used to enhance the sensitivity and specificity of theICD such as, for example, a subcutaneous-only implantable defibrillator.Illustratively, a subcutaneously-only defibrillator senses onlyfar-field signals, making difficult extraction of atrial information aswell as uniquely identifying atrial depolarization from ventriculardepolarization. When a subcutaneous-only defibrillator is used incombination with one or more leadless cardiac pacemakers, theinformation derived from the pacing pulse for each leadless pacemakercan be gathered and used to identify atrial and ventriculardepolarization without ambiguity.

The leadless cardiac pacemaker 102 can communicate the informationlisted hereinabove with the implanted ICD 106, or with a programmeroutside the body, or both.

For example, in some embodiments an individual pacemaker 102 of the oneor more leadless cardiac pacemakers can be configured to deliver a codedpacing pulse with a code assigned according to pacemaker location andconfigured to transmit a message to one or more other leadless cardiacpacemakers via the coded pacing pulse wherein the code identifies theindividual pacemaker originating an event. The pacemaker or pacemakersreceiving the message are adapted to respond to the message in apredetermined manner depending on type and location of the event.

In some embodiments or conditions, individual pacemakers 102 can delivera coded pacing pulse with a code assigned according to pacemakerlocation and configured to transmit a message to at least one of theleadless cardiac pacemakers via the coded pacing pulse wherein the codeidentifies the individual pacemaker originating an event. The individualpacemakers can be further configured to deliver a pacing pulse inabsence of encoding whereby, for dual-chamber cardiac pacing, a pacingpulse that is not generated in a first cardiac pacemaker that senses thepacing pulse is necessarily generated in a second cardiac pacemaker.Accordingly, neither the use of a code to identify the chambercorresponding to a pacing pulse, nor the use of a code to identify thetype of pulse (whether paced or sensed) is a necessary step in a simplesystem such as a dual chamber pacing system disclosed in thespecification.

Moreover, information communicated on the incoming channel can alsoinclude a message from another leadless cardiac pacemaker signifyingthat the other leadless cardiac pacemaker has sensed a heartbeat or hasdelivered a pacing pulse, and identifies the location of the otherpacemaker. Similarly, information communicated on the outgoing channelcan also include a message to another leadless cardiac pacemaker orpacemakers, or to the ICD, that the sending leadless cardiac pacemakerhas sensed a heartbeat or has delivered a pacing pulse at the locationof the sending pacemaker.

In some embodiments and in predetermined conditions, an individualpacemaker 102 of the one or more leadless cardiac pacemakers can beconfigured to communicate to one or more other implanted pacemakersindication of the occurrence of a sensed heartbeat at the individualpacemaker location via generation of a coded pacing pulse triggered bythe sensed heartbeat in a natural refractory period following the sensedheartbeat.

Referring again to FIGS. 1A and 1B, in various embodiments a cardiacpacing system 100 comprises at least one leadless cardiac pacemaker 102that is configured for implantation in electrical contact with a cardiacchamber 104 and configured to perform cardiac pacing functions incombination with a co-implanted implantable cardioverter-defibrillator(ICD) 106.

An embodiment of a cardiac pacing system 100 comprises an implantablecardioverter-defibrillator (ICD) 106 and at least one leadless cardiacpacemaker 102 configured for implantation in electrical contact with acardiac chamber and for performing cardiac rhythm management functionsin combination with the implantable ICD 106. The implantable ICD 106 andthe one or more leadless cardiac pacemakers 102 configured for leadlessintercommunication by information conduction through body tissue.

In another embodiment, a cardiac pacing system 100 comprises one or moreleadless cardiac pacemaker 102 or pacemakers configured for implantationin electrical contact with a cardiac chamber 104 and configured toperform cardiac pacing functions in combination with a co-implantedimplantable cardioverter-defibrillator (ICD) 106. The leadless cardiacpacemaker or pacemakers 102 are configured to intercommunicate and/or tocommunicate with a non-implanted programmer and/or the implanted ICD 106via two or more electrodes 108 that are used for delivering pacingpulses. The pacemakers 102 are configured for antenna-less and telemetrycoil-less communication.

In a further embodiment, a cardiac pacing system 100 comprises at leastone leadless cardiac pacemaker 102 configured for implantation inelectrical contact with a cardiac chamber 104 and configured to performcardiac pacing functions in combination with a co-implanted implantablecardioverter-defibrillator (ICD) 106. The leadless cardiac pacemaker orpacemakers 102 comprise at least two leadless electrodes 108 configuredfor delivering cardiac pacing pulses, sensing evoked and/or naturalcardiac electrical signals, and transmitting information to theco-implanted ICD 106.

In another example embodiment, a cardiac pacing system 100 comprises atleast one leadless cardiac pacemaker 102 configured for implantation inelectrical contact with a cardiac chamber 104 and configured to performcardiac pacing functions in combination with a co-implanted implantablecardioverter-defibrillator (ICD) 106. The leadless cardiac pacemaker orpacemakers 102 comprising at least two leadless electrodes 108configured for delivering cardiac pacing pulses, sensing evoked and/ornatural cardiac electrical signals, and receiving information from theco-implanted ICD 106.

As shown in the illustrative embodiments, a leadless cardiac pacemaker102 can comprise two or more leadless electrodes 108 configured fordelivering cardiac pacing pulses, sensing evoked and/or natural cardiacelectrical signals, and bidirectionally communicating with theco-implanted ICD 106. A leadless cardiac pacemaker 102 can be configuredto communicate with other pacemakers and/or communicate with anon-implanted programmer via communication that has communication powerrequirements essentially met by power consumed in cardiac pacing. Forexample, the leadless cardiac pacemaker 102 can be configured tocommunicate with other pacemakers and with a non-implanted programmervia communication that has negligible transmission power requirements inaddition to power consumed in cardiac pacing.

Individual pacemakers of the one or more leadless cardiac pacemakers 102can be configured for operation in a particular location and aparticular functionality at manufacture and/or at programming by anexternal programmer. Bidirectional communication among the multipleleadless cardiac pacemakers can be arranged to communicate notificationof a sensed heartbeat or delivered pacing pulse event and encoding typeand location of the event to another implanted pacemaker or pacemakers.The pacemaker or pacemakers receiving the communication decode theinformation and respond depending on location of the receiving pacemakerand predetermined system functionality.

In some embodiments, individual pacemakers 102 of the one or moreleadless cardiac pacemakers can be configured to receive conductedcommunication from a co-implanted cardioverter-defibrillator (ICD) 106that configures the pacemakers 102 to deliver overdrive anti-tachycardiapacing in response to a detected tachyarrhythmia.

Also shown in FIG. 1B, the primary battery 114 has positive terminal 140and negative terminal 142. A suitable primary battery has an energydensity of at least 3 W·h/cc, a power output of 70 microwatts, a volumeless than 1 cubic centimeter, and a lifetime greater than 5 years.

One suitable primary battery uses beta-voltaic technology, licensed toBetaBatt Inc. of Houston, Tex., USA, and developed under a trade nameDEC™ Cell, in which a silicon wafer captures electrons emitted by aradioactive gas such as tritium. The wafer is etched in athree-dimensional surface to capture more electrons. The battery issealed in a hermetic package which entirely contains the low-energyparticles emitted by tritium, rendering the battery safe for long-termhuman implant from a radiological-health standpoint. Tritium has ahalf-life of 12.3 years so that the technology is more than adequate tomeet a design goal of a lifetime exceeding 5 years.

Current from the positive terminal 140 of primary battery 114 flowsthrough a shunt 144 to a regulator circuit 146 to create a positivevoltage supply 148 suitable for powering the remaining circuitry of thepacemaker 102. The shunt 144 enables the battery current monitor 136 toprovide the processor 112 with an indication of battery current drainand indirectly of device health.

The illustrative power supply can be a primary battery 114 such as abeta-voltaic converter that obtains electrical energy fromradioactivity. In some embodiments, the power supply can be selected asa primary battery 114 that has a volume less than approximately 1 cubiccentimeter.

The leadless cardiac pacemaker or pacemakers 102 can be configured todetect a natural cardiac depolarization, time a selected delay interval,and deliver an information-encoded pulse during a refractory periodfollowing the natural cardiac depolarization. By encoding information ina pacing pulse, power consumed for transmitting information is notsignificantly greater than the power used for pacing. Information can betransmitted through the communication channel with no separate antennaor telemetry coil. Communication bandwidth is low with only a smallnumber of bits encoded on each pulse.

In some embodiments, information can be encoded using a technique ofgating the pacing pulse for very short periods of time at specificpoints in the pacing pulse. During the gated sections of the pulse, nocurrent flows through the electrodes of a leadless cardiac pacemaker.Timing of the gated sections can be used to encode information. Thespecific length of a gated segment depends on the programmer's abilityto detect the gated section. A certain amount of smoothing or low-passfiltering of the signal can be expected from capacitance inherent in theelectrode/skin interface of the programmer as well as theelectrode/tissue interface of the leadless cardiac pacemaker. A gatedsegment is set sufficiently long in duration to enable accuratedetection by the programmer, limiting the amount of information that canbe transmitted during a single pacing pulse. Accordingly, a techniquefor communication can comprise generating stimulation pulses onstimulating electrodes of an implanted biostimulator and encodinginformation onto generated stimulation pulses. Encoding information ontothe pulses can comprise gating the stimulation pulses for selecteddurations at selected timed sections in the stimulation pulses wherebygating removes current flow through the stimulating electrodes andtiming of the gated sections encodes the information.

Another method of encoding information on pacing pulses involves varyingthe timing between consecutive pacing pulses in a pulse sequence. Pacingpulses, unless inhibited or triggered, occur at predetermined intervals.The interval between any two pulses can be varied slightly to impartinformation on the pulse series. The amount of information, in bits, isdetermined by the time resolution of the pulse shift. The steps of pulseshifting are generally on the order of microseconds. Shifting pulses byup to several milliseconds does not have an effect on the pacing therapyand cannot be sensed by the patient, yet significant information can betransmitted by varying pulse intervals within the microsecond range. Themethod of encoding information in variation of pulses is less effectiveif many of the pulses are inhibited or triggered. Accordingly, atechnique for communication can comprise generating stimulation pulseson stimulating electrodes of an implanted biostimulator and encodinginformation onto generated stimulation pulses comprising selectivelyvarying timing between consecutive stimulation pulses.

Alternatively or in addition to encoding information in gated sectionsand/or pulse interval, overall pacing pulse width can be used to encodeinformation.

The three described methods of encoding information on pacing pulses canuse the programmer to distinguish pacing pulses from the patient'snormal electrocardiogram, for example by recognition of the specificmorphology of the pacing pulse compared to the R-wave generated duringthe cardiac cycle. For example, the external programmer can be adaptedto distinguish a generated cardiac pacing pulse from a natural cardiacdepolarization in an electrocardiogram by performing comparative patternrecognition of a pacing pulse and an R-wave produced during a cardiaccycle.

In an illustrative embodiment, the primary battery 114 can be selectedto source no more than 70 microwatts instantaneously since a higherconsumption may cause the voltage across the battery terminals tocollapse. Accordingly in one illustrative embodiment the circuitsdepicted in FIG. 1B can be designed to consume no more than a total of64 microwatts. The design avoids usage of a large filtering capacitorfor the power supply or other accumulators such as a supercapacitor orrechargeable secondary cell to supply peak power exceeding the maximuminstantaneous power capability of the battery, components that would addvolume and cost.

In various embodiments, the system can manage power consumption to drawlimited power from the battery, thereby reducing device volume. Eachcircuit in the system can be designed to avoid large peak currents. Forexample, cardiac pacing can be achieved by discharging a tank capacitor(not shown) across the pacing electrodes. Recharging of the tankcapacitor is typically controlled by a charge pump circuit. In aparticular embodiment, the charge pump circuit is throttled to rechargethe tank capacitor at constant power from the battery.

Implantable systems that communicate via long distance radio-frequency(RF) schemes, for example Medical Implant Communication Service (MICS)transceivers, which exhibit a peak power requirement on the order of 10milliwatts, and other RF or inductive telemetry schemes are unable tooperate without use of an additional accumulator. Moreover, even withthe added accumulator, sustained operation would ultimately cause thevoltage across the battery to collapse.

In some embodiments, the controller 112 in one leadless cardiacpacemaker 102 can access signals on the electrodes 108 and can examineoutput pulse duration from another pacemaker for usage as a signaturefor determining triggering information validity and, for a signaturearriving within predetermined limits, activating delivery of a pacingpulse following a predetermined delay of zero or more milliseconds. Thepredetermined delay can be preset at manufacture, programmed via anexternal programmer, or determined by adaptive monitoring to facilitaterecognition of the triggering signal and discriminating the triggeringsignal from noise. In some embodiments or in some conditions, thecontroller 112 can examine output pulse waveform from another leadlesscardiac pacemaker for usage as a signature for determining triggeringinformation validity and, for a signature arriving within predeterminedlimits, activating delivery of a pacing pulse following a predetermineddelay of zero or more milliseconds.

Also shown in FIG. 2, a cylindrical hermetic housing 110 is shown withannular electrodes 108 at housing extremities. In the illustrativeembodiment, the housing 110 can be composed of alumina ceramic whichprovides insulation between the electrodes. The electrodes 108 aredeposited on the ceramic, and are platinum or platinum-iridium.

Several techniques and structures can be used for attaching the housing110 to the interior or exterior wall of cardiac chamber muscle 104.

A helix 226 and slot 228 enable insertion of the device endocardially orepicardially through a guiding catheter. A screwdriver stylet can beused to rotate the housing 110 and force the helix 226 into muscle 104,thus affixing the electrode 108A in contact with stimulable tissue.Electrode 108B serves as an indifferent electrode for sensing andpacing. The helix 226 may be coated for electrical insulation, and asteroid-eluting matrix may be included near the helix to minimizefibrotic reaction, as is known in conventional pacing electrode-leads.

In other configurations, suture holes 224 and 225 can be used to affixthe device directly to cardiac muscle with ligatures, during procedureswhere the exterior surface of the heart is exposed.

Other attachment structures used with conventional cardiacelectrode-leads including tines or barbs for grasping trabeculae in theinterior of the ventricle, atrium, or coronary sinus may also be used inconjunction with or instead of the illustrative attachment structures.

Referring to FIG. 3, a pictorial view shows another embodiment of asingle leadless cardiac pacemaker 102 that can be used in a cardiacpacing system 100 with at least one other pacemaker. The leadlesscardiac pacemaker 102 includes a cylindrical metal housing 310 with anannular electrode 108A and a second electrode 108B. Housing 310 can beconstructed from titanium or stainless steel. Electrode 108A can beconstructed using a platinum or platinum-iridium wire and a ceramic orglass feed-thru to provide electrical isolation from the metal housing.The housing can be coated with a biocompatible polymer such as medicalgrade silicone or polyurethane except for the region outlined byelectrode 108B. The distance between electrodes 108A and 108B should beapproximately 1 cm to optimize sensing amplitudes and pacing thresholds.A helix 226 and slot 228 can be used for insertion of the deviceendocardially or epicardially through a guiding catheter. In addition,suture sleeves 302 and 303 made from silicone can be used to affix tothe device directly to cardiac muscle with ligatures, for example in anepicardial or other application.

Referring to FIG. 4, a typical output-pulse waveform for a conventionalpacemaker is shown. The approximately-exponential decay is due todischarge of a capacitor in the pacemaker through theapproximately-resistive load presented by the electrodes and leads.Typically the generator output is capacitor-coupled to one electrode toensure net charge balance. The pulse duration is shown as T0 and istypically 500 microseconds.

When the depicted leadless pacemaker 102 is used in combination with atleast one other pacemaker or other pulse generator in the cardiac pacingsystem 100 and is generating a pacing pulse but is not optionallysending data for communication, the pacing waveform of the leadlesspacemaker 102 can also resemble the conventional pacing pulse shown inFIG. 4.

Referring to FIG. 5, a time waveform graph depicts an embodiment of anoutput-pacing pulse waveform adapted for communication. The output-pulsewaveform of the illustrative leadless pacemaker 102 is shown during atime when the pacemaker 102 is optionally sending data for communicationand also delivering a pacing pulse, using the same pulse generator 116and electrodes 108 for both functions.

FIG. 5 shows that the pulse generator 102 has divided the output pulseinto shorter pulses 501, 502, 503, 504; separated by notches 505, 506,and 507. The pulse generator 102 times the notches 505, 506, and 507 tofall in timing windows W1, W2, and W4 designated 508, 509, and 511respectively. Note that the pacemaker 102 does not form a notch intiming window W3 designated 510. The timing windows are each shownseparated by a time T1, approximately 100 microseconds in the example.

As controlled by processor 112, pulse generator 116 selectivelygenerates or does not generate a notch in each timing window 508, 509,510, and 511 so that the device 102 encodes four bits of information inthe pacing pulse. A similar scheme with more timing windows can sendmore or fewer bits per pacing pulse. The width of the notches is small,for example approximately 15 microseconds, so that the delivered chargeand overall pulse width, specifically the sum of the widths of theshorter pulses, in the pacing pulse is substantially unchanged from thatshown in FIG. 4. Accordingly, the pulse shown in FIG. 5 can haveapproximately the same pacing effectiveness as that shown in FIG. 4,according to the law of Lapique which is well known in the art ofelectrical stimulation.

In a leadless cardiac pacemaker, a technique can be used to conservepower when detecting information carried on pacing pulses from otherimplanted devices. The leadless cardiac pacemaker can have a receivingamplifier that implements multiple gain settings and uses a low-gainsetting for normal operation. The low-gain setting could beinsufficiently sensitive to decode gated information on a pacing pulseaccurately but could detect whether the pacing pulse is present. If anedge of a pacing pulse is detected during low-gain operation, theamplifier can be switched quickly to the high-gain setting, enabling thedetailed encoded data to be detected and decoded accurately. Once thepacing pulse has ended, the receiving amplifier can be set back to thelow-gain setting. For usage in the decoding operation, the receivingamplifier is configured to shift to the more accurate high-gain settingquickly when activated. Encoded data can be placed at the end of thepacing pulse to allow a maximum amount of time to invoke the high-gainsetting.

As an alternative or in addition to using notches in the stimulationpulse, the pulses can be generated with varying off-times, specificallytimes between pulses during which no stimulation occurs. The variationof off-times can be small, for example less than 10 milliseconds total,and can impart information based on the difference between a specificpulse's off-time and a preprogrammed off-time based on desired heartrate. For example, the device can impart four bits of information witheach pulse by defining 16 off-times centered around the preprogrammedoff-time. FIG. 6 is a graph showing a sample pulse generator outputwhich incorporates a varying off-time scheme. In the figure, time T_(P)represents the preprogrammed pulse timing. Time T_(d) is the delta timeassociated with a single bit resolution for the data sent by the pulsegenerator. The number of T_(d) time increments before or after themoment specified by T_(P) gives the specific data element transmitted.The receiver of the pulse generator's communication has advanceinformation of the time T_(P). The communication scheme is primarilyapplicable to overdrive pacing in which time T_(P) is not dynamicallychanging or altered based on detected beats.

FIG. 5 depicts a technique in which information is encoded in notches inthe pacing pulse. FIG. 6 shows a technique of conveying information bymodulating the off-time between pacing pulses. Alternatively or inaddition to the two illustrative coding schemes, overall pacing pulsewidth can be used to impart information. For example, a paced atrialbeat may exhibit a pulse width of 500 microseconds and an intrinsicatrial contraction can be identified by reducing the pulse width by 30microseconds. Information can be encoded by the absolute pacing pulsewidth or relative shift in pulse width. Variations in pacing pulse widthcan be relatively small and have no impact on pacing effectiveness.

In some embodiments, a pacemaker 102 can use the leadless electrodes 108to communicate bidirectionally among multiple leadless cardiacpacemakers and transmit data including designated codes for eventsdetected or created by an individual pacemaker wherein the codes encodeinformation using pacing pulse width.

To ensure the leadless cardiac pacemaker functions correctly, a specificminimum internal supply voltage is maintained. When pacing tankcapacitor charging occurs, the supply voltage can drop from apre-charging level which can become more significant when the batterynears an end-of-life condition and has reduced current sourcingcapability. Therefore, a leadless cardiac pacemaker can be constructedwith a capability to stop charging the pacing tank capacitor when thesupply voltage drops below a specified level. When charging ceases, thesupply voltage returns to the value prior to the beginning of tankcapacitor charging.

In another technique, the charge current can be lowered to prevent thesupply voltage from dropping below the specified level. However,lowering the charge current can create difficulty in ensuring pacingrate or pacing pulse amplitude are maintained, since the lower chargecurrent can extend the time for the pacing tank capacitor to reach atarget voltage level.

The illustrative scheme for transmitting data does not significantlyincrease the current consumption of the pacemaker. For example, thepacemaker could transmit data continuously in a loop, with noconsumption penalty.

The illustrative schemes for transmitting data enable assignment ofdesignated codes to events detected or caused by a leadless cardiacpacemaker, such as sensing a heartbeat or delivering a pacing pulse atthe location of the pacemaker that senses the event. Individual leadlesscardiac pacemakers 102 in a system 100 can be configured, either atmanufacture or with instructions from an external programmer or from theco-implanted ICD 106 as described hereinabove, to issue a unique codecorresponding to the type of event and location of the leadless cardiacpacemaker. By delivery of a coded pacing pulse with a code assignedaccording to the pacemaker location, a leadless cardiac pacemaker cantransmit a message to any and all other leadless cardiac pacemakers 102and the ICD 106 implanted in the same patient, where the code signifiesthe origin of the event. Each other leadless cardiac pacemaker can reactappropriately to the conveyed information in a predetermined mannerencoded in the internal processor 112, as a function of the type andlocation of the event coded in the received pulse. The ICD 106 can alsouse the information for arrhythmia detection. A leadless cardiacpacemaker 102 can thus communicate to any and all other co-implantedleadless cardiac pacemakers and to the co-implanted ICD 106 theoccurrence of a sensed heartbeat at the originating pacemaker's locationby generating a coded pacing pulse triggered by the sensed event.Triggered pacing occurs in the natural refractory period following theheartbeat and therefore has no effect on the chamber where the leadlesscardiac pacemaker is located.

Referring again to FIG. 1B, the circuit 132 for receiving communicationvia electrodes 108 receives the triggering information as described andcan also optionally receive other communication information, either fromthe other implanted pulse generator 106 or from a programmer outside thebody. This other communication could be coded with a pulse-positionscheme as described in FIG. 5 or could otherwise be a pulse-modulated orfrequency-modulated carrier signal, preferably from 10 kHz to 100 kHz.The illustrative scheme of a modulated carrier is applicable not only tointercommunication among multiple implanted pacemakers but also isapplicable to communication from an external programmer or theco-implanted ICD 106.

The illustrative leadless pacemaker 102 could otherwise receivetriggering information from the other pulse generator 106 implantedwithin the body via a pulse-modulated or frequency-modulated carriersignal, instead of via the pacing pulses of the other pulse generator106.

With regard to operating power requirements in the leadless cardiacpacemaker 102, for purposes of analysis, a pacing pulse of 5 volts and 5milliamps amplitude with duration of 500 microseconds and a period of500 milliseconds has a power requirement of 25 microwatts.

In an example embodiment of the leadless pacemaker 102, the processor112 typically includes a timer with a slow clock that times a period ofapproximately 10 milliseconds and an instruction-execution clock thattimes a period of approximately 1 microsecond. The processor 112typically operates the instruction-execution clock only briefly inresponse to events originating with the timer, communication amplifier134, or cardiac sensing amplifier 132. At other times, only the slowclock and timer operate so that the power requirement of the processor112 is no more than 5 microwatts.

For a pacemaker that operates with the aforementioned slow clock, theinstantaneous power consumption specification, even for acommercially-available micropower microprocessor, would exceed thebattery's power capabilities and would require an additional filtercapacitor across the battery to prevent a drop of battery voltage belowthe voltage necessary to operate the circuit. The filter capacitor wouldadd avoidable cost, volume, and potentially lower reliability.

For example, a microprocessor consuming only 100 microamps would requirea filter capacitor of 5 microfarads to maintain a voltage drop of lessthan 0.1 volt, even if the processor operates for only 5 milliseconds.To avoid the necessity for such a filter capacitor, an illustrativeembodiment of a processor can operate from a lower frequency clock toavoid the high instantaneous power consumption, or the processor can beimplemented using dedicated hardware state machines to supply a lowerinstantaneous peak power specification.

In a pacemaker, the cardiac sensing amplifier typically operates with nomore than 5 microwatts. A communication amplifier at 100 kHz operateswith no more than 25 microwatts. The battery ammeter and batteryvoltmeter operate with no more than 1 microwatt each.

A pulse generator typically includes an independent rate limiter with apower consumption of no more than 2 microwatts.

The total power consumption of the pacemaker is thus 64 microwatts, lessthan the disclosed 70-microwatt battery output.

Improvement attained by the illustrative cardiac pacing system 100 andleadless cardiac pacemaker 102 is apparent.

In a specific embodiment, the outgoing communication power requirementplus the pacing power requirement does not exceed approximately 25microwatts. In other words, outgoing communication adds essentially nopower to the power used for pacing.

The illustrative leadless cardiac pacemaker 102 can have sensing andprocessing circuitry that consumes no more than 10 microwatts as inconventional pacemakers.

The described leadless cardiac pacemaker 102 can have an incomingcommunication amplifier for receiving triggering signals and optionallyother communication which consumes no more than 25 microwatts.

Furthermore, the leadless cardiac pacemaker 102 can have a primarybattery that exhibits an energy density of at least 3 watt-hours percubic centimeter (W·h/cc).

In an illustrative application of the cardiac pacing system 100, one ormore leadless cardiac pacemakers 102 can be co-implanted with an ICD 106in a single patient to provide a system for single-chamber pacing,dual-chamber pacing, CRT-D, or any other multi-chamber pacingapplication. Each leadless cardiac pacemaker in the system can use theillustrative communication structures to communicate the occurrence of asensed heartbeat or a delivered pacing pulse at the location of sensingor delivery, and a communication code can be assigned to eachcombination of event type and location. Each leadless cardiac pacemakercan receive the transmitted information, and the code of the informationcan signify that a paced or sensed event has occurred at anotherlocation and indicate the location of occurrence. The receiving leadlesscardiac pacemaker's processor 112 can decode the information and respondappropriately, depending on the location of the receiving pacemaker andthe desired function of the system.

The implanted cardioverter-defibrillator (ICD) 106 can comprise a caseand be fitted with a pair of electrodes mounted on or near the case. TheICD 106 can be configured to receive and transmit conductedcommunication using a pulse modulated or frequency modulated carriersignal whereby the ICD 106 can detect communication pulses fromco-implanted leadless cardiac pacemakers 102 and transmit programminginformation to the co-implanted leadless cardiac pacemakers 102. In someembodiments, an implanted cardioverter-defibrillator (ICD) 106configured to receive conducted communication using two implantableelectrodes.

FIGS. 7 and 8 are state diagrams that illustrate application ofillustrative combined control operations in an atrial andright-ventricular leadless cardiac pacemaker respectively, to implementa simple dual-chamber pacing system when co-implanted with an ICD 106.FIG. 9 is a state diagram that illustrates inclusion of aleft-ventricular leadless cardiac pacemaker to form a CRT-D system. Invarious embodiments, each leadless cardiac pacemaker may also broadcastother information destined for co-implanted leadless cardiac pacemakersand the co-implanted ICD, besides markers of paced or sensed events.

For clarity of illustration, descriptions of the atrial, rightventricular, and left-ventricular leadless cardiac pacemakers inrespective FIGS. 7, 8, and 9 show only basic functions of eachpacemaker. Other functions such as refractory periods, fallback modeswitching, algorithms to prevent pacemaker-mediated tachycardia, and thelike, can be added to the leadless cardiac pacemakers and to the systemin combination. Also for clarity, functions for communication with anexternal programmer are not shown and are shown elsewhere herein.

Referring to FIG. 7, a state-mechanical representation shows operationof a leadless cardiac pacemaker for implantation adjacent to atrialcardiac muscle. As explained above, a leadless cardiac pacemaker can beconfigured for operation in a particular location and system either atmanufacture or by an external programmer. Similarly, all individualpacemakers of the multiple pacemaker system can be configured foroperation in a particular location and a particular functionality atmanufacture and/or at programming by an external programmer wherein“configuring” means defining logic such as a state machine and pulsecodes used by the leadless cardiac pacemaker.

In a cardiac pacing system, the multiple leadless cardiac pacemakers cancomprise an atrial leadless cardiac pacemaker implanted in electricalcontact to an atrial cardiac chamber. The atrial leadless cardiacpacemaker can be configured or programmed to perform several controloperations 700 in combination with one or more other pacemakers. In await state 702 the atrial leadless cardiac pacemaker waits for anearliest occurring event of multiple events including a sensed atrialheartbeat 704, a communication of an event sensed on the at least twoleadless electrodes encoding a pacing pulse marking a heartbeat 706 at aventricular leadless cardiac pacemaker, or timeout of an interval timedlocally in the atrial leadless cardiac pacemaker shown as escapeinterval timeout 708. The atrial pacemaker responds to a sensed atrialheartbeat 704 by generating 710 an atrial pacing pulse that signals toone or more other pacemakers and optionally to the co-implanted ICD thatan atrial heartbeat has occurred, encoding the atrial pacing pulse witha code signifying an atrial location and a sensed event type. The atrialpacing pulse can be encoded using the technique shown in FIG. 5 with aunique code signifying the location in the atrium. After pacing theatrium, the atrial cardiac pacemaker times 712 a predeterminedatrial-to-atrial (AA) escape interval. Accordingly, the atrial leadlesscardiac pacemaker restarts timing 712 for a predetermined escapeinterval, called the AA (atrial to atrial) escape interval, which is thetime until the next atrial pacing pulse if no other event intervenes.The atrial leadless cardiac pacemaker then re-enters the Wait state 702.The atrial pacemaker also responds to timeout of a first occurringescape interval 708 by delivering an atrial pacing pulse 710, causing anatrial heartbeat with the atrial pacing pulse encoding paced type andatrial location of an atrial heartbeat event. When the atrial escapeinterval times out, shown as transition 708, the atrial leadless cardiacpacemaker delivers an atrial pacing pulse. Because no other atrialheartbeat has occurred during the duration of the escape interval, theatrial pacing pulse does not fall in the atria's natural refractoryperiod and therefore should effectively pace the atrium, causing anatrial heartbeat. The atrial pacing pulse, coded in the manner shown inFIG. 5, also signals to any and all other co-implanted leadless cardiacpacemakers and optionally to the co-implanted ICD that an atrialheartbeat has occurred. If functionality is enhanced for a more complexsystem, the atrial leadless cardiac pacemaker can use a different codeto signify synchronous pacing triggered by an atrial sensed event incomparison to the code used to signify atrial pacing at the end of anescape interval. However, in the simple example shown in FIGS. 7 and 8,the same code can be used for all atrial pacing pulses. In fact, for thesimple dual-chamber pacing system described in FIGS. 7 and 8 encodingmay be omitted because each leadless cardiac pacemaker can conclude thatany detected pacing pulse, which is not generated locally, must haveoriginated with the other co-implanted leadless cardiac pacemaker. Aftergenerating the atrial pacing pulse 710, the atrial leadless cardiacpacemaker starts timing an atrial (AA) escape interval at action 712,and then returns to the wait state 702.

The atrial leadless cardiac pacemaker can further operate in response toanother pacemaker. The atrial pacemaker can detect 706 a signaloriginating from a co-implanted ventricular leadless cardiac pacemaker.The atrial pacemaker can examine the elapsed amount of theatrial-to-atrial (AA) escape time interval since a most recent atrialheartbeat and determine 714 whether the signal originating from theco-implanted ventricular leadless cardiac pacemaker is premature. Thus,if the atrial leadless cardiac pacemaker detects a signal originatingfrom a co-implanted ventricular leadless cardiac pacemaker, shown assensed ventricular pacing 706, then the atrial device examines theamount of the escape interval elapsed since the last atrial heartbeat atdecision point 714 to determine whether the ventricular event is“premature”, meaning too late to be physiologically associated with thelast atrial heartbeat and in effect premature with respect to the nextatrial heartbeat. In the absence 716 of a premature signal, the atrialpacemaker waits 702 for an event with no effect on atrial pacing. Incontrast if the signal is premature 718, the pacemaker restarts 720 aventricle-to-atrium (VA) escape interval that is shorter than theatrial-to-atrial (AA) escape interval and is representative of a typicaltime from a ventricular beat to a next atrial beat in sinus rhythm,specifically the atrial interval minus the atrio-ventricular conductiontime. After starting 720 the VA interval, the atrial leadless cardiacpacemaker returns to wait state 702, whereby a ventricular prematurebeat can be said to “recycle” the atrial pacemaker. The pacemakerresponds to timeout of the atrial-to-atrial (AA) escape interval 708 bydelivering an atrial pacing pulse 710, causing an atrial heartbeat. Theatrial pacing pulse encodes the paced type and atrial location of anatrial heartbeat event.

The atrial leadless cardiac pacemaker can be further configured to timea prolonged post-ventricular atrial refractory period (PVARP) afterrecycling in presence of the premature signal, thereby preventingpacemaker-mediated tachycardia (PMT). Otherwise, if a receivedventricular pacing signal evaluated at decision point 714 is not foundto be premature, then the atrial leadless cardiac pacemaker followstransition 716 and re-enters the wait state 702 without recycling, thuswithout any effect on the timing of the next atrial pacing pulse.

Referring to FIG. 8, a state-mechanical representation depicts operationof a leadless cardiac pacemaker for implantation adjacent toright-ventricular cardiac muscle. The leadless cardiac pacemaker can beconfigured for operation in a particular location and system either atmanufacture or by an external programmer. A system comprising multipleleadless cardiac pacemakers can include a right-ventricular leadlesscardiac pacemaker implanted in electrical contact to a right-ventricularcardiac chamber. The right-ventricular leadless cardiac pacemaker can beconfigured to perform actions 800 for coordinated pacing in combinationwith the other pacemakers. The right-ventricular leadless cardiacpacemaker waits 802 for the earliest occurring event of multiple eventsincluding a sensed right-ventricular heartbeat 804, a sensedcommunication of a pacing pulse 806 marking a heartbeat at an atrialleadless cardiac pacemaker, and timeout 808 of an escape interval.Generally, the sensed communication of a pacing pulse 806 can be anysuitable sensed communication of an event originating at anotherco-implanted leadless cardiac pacemaker, in the illustrative embodimenta pacing pulse marking a heartbeat at an atrial leadless cardiacpacemaker shown as sensed atrial pacing. The escape interval timeout 808can be any suitable timeout of an interval timed locally in theright-ventricular leadless cardiac pacemaker.

The right-ventricular leadless cardiac pacemaker responds to the sensedright-ventricular heartbeat 804 by generating 810 a right-ventricularpacing pulse that signals to at least one other pacemaker of themultiple cardiac pacemakers and optionally to the co-implanted ICD thata right-ventricular heartbeat has occurred. Thus, when a sensedright-ventricular heartbeat occurs 804, the right-ventricular leadlesscardiac pacemaker generates 810 a right-ventricular pacing pulse, not topace the heart but rather to signal to another leadless cardiacpacemaker or pacemakers that a right-ventricular heartbeat has occurred.The right-ventricular pacing pulse can be encoded with a code signifyingthe right-ventricular location and a sensed event type. Theright-ventricular pacing pulse is coded in the manner shown in FIG. 5with a unique code signifying the location in the right ventricle. Uponright-ventricular pacing pulse generation 810, the right-ventricularleadless cardiac pacemaker can time 812 a predetermined rightventricular-to-right ventricular (VV) escape interval. Theright-ventricular leadless cardiac pacemaker restarts 812 timing of apredetermined escape interval, called the VV (right) (right-ventricularto right-ventricular) escape interval, which is the time until the nextright-ventricular pacing pulse if no other event intervenes.

The right-ventricular leadless cardiac pacemaker can further beconfigured to set the ventricular-to-ventricular (VV) escape intervallonger than a predetermined atrial-to-atrial (AA) escape interval toenable backup ventricular pacing at a low rate corresponding to the VVescape interval in case of failure of a triggered signal from aco-implanted atrial leadless cardiac pacemaker. Typically, the VV(right) escape interval is longer than the AA interval depicted in FIG.7, so that the system supports backup ventricular pacing at a relativelylow rate in case of failure of the co-implanted atrial leadless cardiacpacemaker. In normal operation of the system, timeout of the VV intervalnever occurs. The right-ventricular leadless cardiac pacemaker thenre-enters the Wait state 802.

The right-ventricular leadless cardiac pacemaker can respond to timeoutof a first occurring escape interval 808 by delivering 810 a rightventricular pacing pulse, causing a right ventricular heartbeat. Theright ventricular pacing pulse can encode information including pacedtype and right-ventricular location of a right ventricular heartbeatevent.

When the right-ventricular escape interval times out 808, theright-ventricular leadless cardiac pacemaker delivers 810 aright-ventricular pacing pulse. Because no other right-ventricularheartbeat has occurred during the duration of the VV escape interval,the pacing pulse 810 does not fall in the ventricles' natural refractoryperiod and therefore should effectively pace the ventricles, causing aventricular heartbeat. The right-ventricular pacing pulse, coded in themanner shown in FIG. 5, also signals to any and all other co-implantedleadless cardiac pacemakers and optionally to the co-implanted ICD thata right-ventricular heartbeat has occurred. If useful for the functionof a more complex system, the right-ventricular leadless cardiacpacemaker can use a different code to signify synchronous pacingtriggered by a right-ventricular sensed event in comparison to the codeused to signify right-ventricular pacing at the end of a VV escapeinterval. However, in the simple example shown in FIGS. 7 and 8, thesame code can be used for all right-ventricular pacing pulses. In fact,for the simple dual-chamber pacing system described in FIGS. 7 and 8, acode may be omitted because each leadless cardiac pacemaker can concludethat any detected pacing pulse which is not generated local to thepacemaker originates with the other co-implanted leadless cardiacpacemaker. After generating 810 the right-ventricular pacing pulse, theright-ventricular leadless cardiac pacemaker starts timing 812 aright-ventricular escape interval VV, and then returns to the wait state802.

The right-ventricular leadless cardiac pacemaker can further beconfigured to detect 806 a signal originating from a co-implanted atrialleadless cardiac pacemaker. The right-ventricular leadless cardiacpacemaker examines the elapsed amount of the ventricular-to-ventricular(VV) escape interval since a most recent right-ventricular heartbeat anddetermines 814 whether the signal originating from the co-implantedatrial leadless cardiac pacemaker is premature. An atrial event isdefined as premature if too early to trigger an atrio-ventricular delayto produce a right-ventricular heartbeat. In the presence of a prematuresignal 816, the right-ventricular leadless cardiac pacemaker returns tothe wait state 802 with no further action. Thus, a premature atrial beatdoes not affect ventricular pacing. In the absence of a premature signal818, the right-ventricular leadless cardiac pacemaker starts 820 a rightatrium to right ventricular (AV) escape interval that is representativeof a typical time from an atrial beat to a right-ventricular beat insinus rhythm. Thus a non-premature atrial event leads to starting 820 anAV (right) atrium to right-ventricular escape interval that represents atypical time from an atrial beat to a right-ventricular beat innormally-conducted sinus rhythm. After starting 820 the AV interval, theright-ventricular leadless cardiac pacemaker returns to the wait state802 so that a non-premature atrial beat can “trigger” theright-ventricular pacemaker after a physiological delay. Theright-ventricular leadless cardiac pacemaker also responds to timeout ofeither the VV escape interval and the AV escape interval 808 bydelivering 810 a right ventricular pacing pulse, causing a rightventricular heartbeat. The right ventricular pacing pulse encodes pacedtype and right-ventricular location of a right ventricular heartbeatevent.

Accordingly, co-implanted atrial and right-ventricular leadless cardiacpacemakers depicted in FIGS. 7 and 8 cooperate to form a dual-chamberpacing system.

Referring to FIG. 9, a state-mechanical representation illustrates theoperation of a leadless cardiac pacemaker for implantation adjacent toleft-ventricular cardiac muscle. The left-ventricular cardiac pacemakercan be used in combination with the dual-chamber pacemaker that includesthe atrial leadless cardiac pacemaker and the right-ventricular leadlesscardiac pacemaker described in FIGS. 7 and 8 respectively to form asystem for CRT-D. A leadless cardiac pacemaker, for example theleft-ventricular cardiac pacemaker, can be configured for operation in aparticular location and system either at manufacture or by an externalprogrammer.

A cardiac pacing system, such as a CRT-D system, can include multipleleadless cardiac pacemakers including a left-ventricular leadlesscardiac pacemaker implanted in electrical contact to a left-ventricularcardiac chamber. The left-ventricular leadless cardiac pacemaker canexecute operations of an illustrative pacing method 900. In a wait state902, the left-ventricular cardiac pacemaker waits 902 at theleft-ventricular leadless cardiac pacemaker for an earliest occurringevent of multiple events including a sensed communication 904 of apacing pulse marking a heartbeat at an atrial leadless cardiac pacemakerand timeout 906 of a left ventricular escape interval. Generally, thesensed communication 904 can be the sensed communication of an eventoriginating at another co-implanted leadless cardiac pacemaker, in theillustrative embodiment a pacing pulse marking a heartbeat at an atrialleadless cardiac pacemaker shown as sensed atrial pacing. The escapeinterval timeout 906 can be timeout of an interval timed locally in theleft-ventricular leadless cardiac pacemaker. In the wait state 902 forthe left-ventricular leadless cardiac pacemaker, operation is simplifiedand the left-ventricular pacemaker does not respond to left-ventricularheartbeats. Also, the left-ventricular cardiac pacemaker does not pacethe left ventricle in the absence of a triggering signal from the atrialleadless cardiac pacemaker. The left-ventricular cardiac pacemakerresponds to timeout 906 of the left ventricular escape interval bydelivering 908 a left ventricular pacing pulse, causing a leftventricular heartbeat. The left ventricular pacing pulse encodes thetype and location of a left ventricular heartbeat event. Theleft-ventricular pacing pulse can be coded in the manner shown in FIG. 5to communicate signals to any and all other co-implanted leadlesscardiac pacemakers and optionally to the co-implanted ICD that aleft-ventricular heartbeat has occurred, although such encoding is notnecessary in the simplified CRT-D system shown in the describedembodiment because the other leadless cardiac pacemakers do not react toleft-ventricular pacing. After generating 908 the left-ventricularpacing pulse, the left-ventricular leadless cardiac pacemaker returns tothe wait state 902.

The left-ventricular leadless cardiac pacemaker can be furtherconfigured detect a signal originating from a co-implanted atrialleadless cardiac pacemaker and examine the elapsed amount of the leftventricular escape interval since a most recent left-ventricularheartbeat. The left-ventricular cardiac pacemaker can determine 910whether the signal originating from the co-implanted atrial leadlesscardiac pacemaker is premature. If the left-ventricular leadless cardiacpacemaker detects sensed atrial pacing, then the left-ventricular devicedetermines whether the atrial event is premature, meaning too early totrigger an atrio-ventricular delay to produce a left-ventricularheartbeat. In the presence of a premature signal 912, theleft-ventricular cardiac pacemaker reverts to the wait state 902 andwaits for an event with no effect on ventricular pacing so that apremature atrial beat does not affect ventricular pacing. In absence ofa premature signal 914, the left-ventricular cardiac pacemaker starts916 a left atrium to left ventricular (AV) escape interval that isrepresentative of a typical time from an atrial beat to a leftventricular beat in normally-conducted sinus rhythm. As shown in thedepicted embodiment, the AV (left) escape interval can have a differentvalue from the AV (right) escape interval. After starting 916 the AVinterval, the left-ventricular leadless cardiac pacemaker returns towait state 902. Accordingly, a non-premature atrial beat can “trigger”the left-ventricular pacemaker after a physiological delay.

The left-ventricular cardiac pacemaker also responds to timeout 906 ofthe AV escape interval by delivering 908 a left ventricular pacingpulse, causing a left ventricular heartbeat. The left ventricular pacingpulse encodes paced type and left ventricular location of a leftventricular heartbeat event.

In various embodiments, the multiple leadless cardiac pacemakers cancomprise a right ventricular leadless cardiac pacemaker and a leftventricular leadless cardiac pacemaker that are configured to operatewith atrio-ventricular (AV) delays whereby a left ventricular pacingpulse can be delivered before, after, or substantially simultaneouslywith a right ventricular pacing pulse. For example, multipleco-implanted leadless cardiac pacemakers that function according to thestate diagrams shown in FIGS. 7, 8, and 9 can support CRT-D withleft-ventricular pacing delivered before, at the same time as, or afterright-ventricular pacing.

The co-implanted ICD can configure the leadless cardiac pacemakers viaconducted communication in a similar manner to an external programmer.In particular, the ICD can configure them to deliver overdriveanti-tachycardia pacing in response to detected tachyarrhythmias.

In various embodiments, multiple co-implanted leadless cardiacpacemakers can be configured for multi-site pacing that synchronizesdepolarization for tachyarrhythmia prevention.

The illustrative system can be useful in conjunction with an ICD, andmore particularly with a subcutaneous ICD, for such an ICD has no othermeans to provide bradycardia support, anti-tachycardia pacing, and CRT.

Referring to FIGS. 10A, 10B, 11A, 11B, 12A, and 12B, schematic flowcharts illustrate an embodiment of a method for operating a cardiacpacing system that comprises an implantable cardioverter-defibrillator(ICD) and one or more leadless cardiac pacemakers configured forimplantation in electrical contact with a cardiac chamber and configuredfor performing cardiac pacing functions in combination with the ICD.Pacing functions include delivering cardiac pacing pulses, sensingevoked and/or natural cardiac electrical signals, and bidirectionallycommunicating with a co-implanted ICD and/or at least one otherpacemaker. The one or more leadless cardiac pacemakers are furtherconfigured to communicate a code that signifies occurrence of sensedcardiac electrical signals and/or delivered pacing pulses and identifiesan event type and/or location.

Two or more electrodes are coupled to the ICD and configured to transmitand/or receive conducted communication using a pulse-modulated orfrequency-modulated carrier signal. The ICD can be configured to detectcommunication pulses from at least one co-implanted leadless cardiacpacemaker and transmit programming information to the at least oneco-implanted leadless cardiac pacemaker.

The leadless cardiac pacemakers can be configured to broadcastinformation to the co-implanted ICD and/or at least one other pacemaker.The leadless cardiac pacemakers can further be configured to receive thecode and react based on the code, location of the receiving leadlesscardiac pacemaker, and predetermined system functionality.

In various embodiments, configurations, and conditions, the leadlesscardiac pacemakers can be adapted to perform one or more cardiac pacingfunctions such as single-chamber pacing, dual-chamber pacing, cardiacresynchronization therapy with cardioversion/defibrillation (CRT-D),single-chamber overdrive pacing for prevention of tachyarrhythmias,single-chamber overdrive pacing for conversion of tachyarrhythmias,multiple-chamber pacing for prevention of tachyarrhythmias,multiple-chamber pacing for conversion of tachyarrhythmias, and thelike.

Multiple leadless cardiac pacemakers can be configured forco-implantation in a single patient and multiple-chamber pacingincluding CRT-D. Bidirectional communication among the multiple leadlesscardiac pacemakers can be adapted to communicate notification of asensed heartbeat or delivered pacing pulse event and encoding type andlocation of the event to at least one pacemaker of the leadless cardiacpacemaker plurality. The one or more pacemakers that receive thecommunication can decode the information and react depending on locationof the receiving pacemaker and predetermined system functionality.

FIG. 10A depicts a method 1000 for operating one or more leadlesscardiac pacemakers including an atrial leadless cardiac pacemaker thatis implanted in electrical contact to an atrial cardiac chamber andconfigured for dual-chamber pacing in combination with the co-implantedICD. Cardiac pacing comprises configuring 1002 a multiple leadlesscardiac pacemakers for implantation and configuring 1004 an atrialleadless cardiac pacemaker of the multiple leadless cardiac pacemakersfor implantation in electrical contact to an atrial cardiac chamber. Theatrial leadless cardiac pacemaker waits 1006 for an earliest occurringevent of multiple events including a sensed atrial heartbeat, acommunication of an event sensed on the at least two leadless electrodesencoding a pacing pulse marking a heartbeat at a ventricular leadlesscardiac pacemaker, and timeout of an atrial-to-atrial (AA) escapeinterval. The atrial leadless cardiac pacemaker responds 1008 to thesensed atrial heartbeat by generating an atrial pacing pulse thatsignals to at least one pacemaker of the multiple leadless cardiacpacemakers and optionally to the co-implanted ICD that an atrialheartbeat has occurred and that encodes the atrial pacing pulse with acode signifying an atrial location and a sensed event type. After eithera sensed atrial heartbeat or timeout of an escape interval, the atrialleadless cardiac pacemaker delivers 1010 an atrial pacing pulse, causingan atrial heartbeat and starts 1012 timing a predetermined length AAescape interval, then waiting 1006 for an event. The atrial pacing pulseidentifies paced type and/or atrial location of an atrial heartbeatevent.

In some embodiments, the atrial leadless cardiac pacemaker can encode anatrial pacing pulse that identifies synchronous pacing triggered by anatrial sensed event with a first code and encode an atrial pacing pulsethat identifies atrial pacing following the AA escape interval with asecond code distinct from the first code.

The atrial leadless cardiac pacemaker can, upon delivery of an atrialpacing pulse, time an atrial-to-atrial (AA) escape interval.

FIG. 10B is a flow chart showing another aspect 1050 of the methodembodiment for operating an atrial leadless cardiac pacemaker. Theatrial leadless cardiac pacemaker detects 1052 a signal originating froma co-implanted ventricular leadless cardiac pacemaker and examines 1054an elapsed amount of the atrial-to-atrial (AA) escape interval since amost recent atrial heartbeat, determining 1056 whether the signaloriginating from the co-implanted ventricular leadless cardiac pacemakeris premature. In absence of a premature signal 1058, the atrial leadlesscardiac pacemaker waits 1060 for an event with no effect on atrialpacing, returning to wait state 1006. In presence of a premature signal1062, the atrial leadless cardiac pacemaker restarts 1064 aventricle-to-atrium (VA) escape interval that is shorter than theatrial-to-atrial (AA) escape interval and representative of a typicaltime from a ventricular beat to a next atrial beat in sinus rhythm, thenreturns to wait state 1006.

Referring to FIGS. 11A and 11B, schematic flow charts illustrate anembodiment of a method for operating a right-ventricular leadlesscardiac pacemaker in an illustrative multi-chamber cardiac pacingsystem. The right-ventricular leadless cardiac pacemaker is implanted inelectrical contact to a right-ventricular cardiac chamber and configuredfor dual-chamber pacing in combination with the co-implanted ICD. FIG.11A depicts a method 1100 for cardiac pacing comprising configuring 1102a plurality of leadless cardiac pacemakers for implantation andconfiguring 1104 a right-ventricular leadless cardiac pacemaker of themultiple leadless cardiac pacemakers for implantation in electricalcontact to a right-ventricular cardiac chamber. The right-ventricularleadless cardiac pacemaker waits 1106 for an earliest occurring event ofa several events including a sensed right-ventricular heartbeat, asensed communication of a pacing pulse marking a heartbeat at an atrialleadless cardiac pacemaker, and timeout of an escape interval. Theright-ventricular leadless cardiac pacemaker responds 1108 to the sensedright-ventricular heartbeat by generating a right-ventricular pacingpulse that signals to at least one pacemaker of the leadless cardiacpacemakers and optionally to the co-implanted ICD that aright-ventricular heartbeat has occurred and that encodes theright-ventricular pacing pulse with a code signifying aright-ventricular location and a sensed event type. Theright-ventricular leadless cardiac pacemaker responds 1110 to timeout ofa first-occurring escape interval by delivering a right ventricularpacing pulse, causing a right ventricular heartbeat, with the rightventricular pacing pulse encoding paced type and right ventricularlocation of a right ventricular heartbeat event, and times 1112 apredetermined ventricular-to-ventricular (VV) escape interval.

In some embodiments, the right-ventricular leadless cardiac pacemakercan encode a right-ventricular pacing pulse that identifies synchronouspacing triggered by a right-ventricular sensed event with a first codeand encode a right-ventricular pacing pulse that identifiesright-ventricular pacing following a ventricular-to-ventricular (VV)escape interval with a second code distinct from the first code.

In some embodiments, the right-ventricular leadless cardiac pacemaker,upon delivery of a right-ventricular pacing pulse, can time aventricular-to-ventricular (VV) escape interval.

FIG. 11B is a flow chart showing another aspect of an embodiment of amethod 1150 for operating a right-ventricular leadless cardiacpacemaker. The right-ventricular leadless cardiac pacemaker detects 1152a signal originating from a co-implanted atrial leadless cardiacpacemaker, examines 1154 the elapsed amount of theventricular-to-ventricular (VV) escape interval since a most recentright-ventricular heartbeat, and determines 1156 whether the signaloriginating from the co-implanted atrial leadless cardiac pacemaker ispremature. In presence 1158 of a premature signal, the right-ventricularleadless cardiac pacemaker waits 1160 for an event with no effect onventricular pacing, returning to wait state 1106. In absence 1162 of apremature signal, the right-ventricular leadless cardiac pacemakerstarts 1164 a right atrium to right ventricular (AV) escape intervalthat is representative of a typical time from an atrial beat to aright-ventricular beat in sinus rhythm, and then returns to the waitstate 1106.

Referring to FIGS. 12A and 12B, schematic flow charts illustrateembodiments of a method for operating a left-ventricular leadlesscardiac pacemaker in multi-chamber cardiac pacing system. Theleft-ventricular leadless cardiac pacemaker is implanted in electricalcontact to a left-ventricular cardiac chamber and configured fordual-chamber pacing in combination with the co-implanted ICD. FIG. 12Adepicts a method 1200 for cardiac pacing comprising configuring 1202 aplurality of leadless cardiac pacemakers for implantation andconfiguring 1204 a left-ventricular leadless cardiac pacemaker of theleadless cardiac pacemaker plurality for implantation in electricalcontact to a left-ventricular cardiac chamber and for operation incardiac resynchronization therapy (CRT-D). The left-ventricular cardiacpacemaker waits 1206 at the left-ventricular leadless cardiac pacemakerfor an earliest occurring event of a plurality of events comprising asensed communication of a pacing pulse marking a heartbeat at an atrialleadless cardiac pacemaker and timeout of a left ventricular escapeinterval. The left-ventricular cardiac pacemaker responds 1208 totimeout of the left ventricular escape interval by delivering a leftventricular pacing pulse, causing a left ventricular heartbeat, the leftventricular pacing pulse encoding type and location of a leftventricular heartbeat event.

In some embodiments, the left-ventricular cardiac pacemaker canconfigure the left-ventricular leadless cardiac pacemaker for operationin cardiac resynchronization therapy (CRT-D).

FIG. 12B is a flow chart showing another embodiment of a method 1250 foroperating a left-ventricular leadless cardiac pacemaker. Theleft-ventricular leadless cardiac pacemaker detects 1252 a signaloriginating from a co-implanted atrial leadless cardiac pacemaker,examines 1254 the elapsed amount of the left ventricular escape intervalsince a most recent left-ventricular heartbeat, and determines 1256whether the signal originating from the co-implanted atrial leadlesscardiac pacemaker is premature. In the presence 1258 of a prematuresignal, the left-ventricular cardiac pacemaker waits 1260 for an eventwith no effect on ventricular pacing. In the absence 1262 of a prematuresignal, the left-ventricular cardiac pacemaker starts 1264 a left atriumto left ventricular (AV) escape interval that is representative of atypical time from an atrial beat to a left ventricular beat in sinusrhythm.

Terms “substantially”, “essentially”, or “approximately”, that may beused herein, relate to an industry-accepted tolerance to thecorresponding term. Such an industry-accepted tolerance ranges from lessthan one percent to twenty percent and corresponds to, but is notlimited to, component values, integrated circuit process variations,temperature variations, rise and fall times, and/or thermal noise. Theterm “coupled”, as may be used herein, includes direct coupling andindirect coupling via another component, element, circuit, or modulewhere, for indirect coupling, the intervening component, element,circuit, or module does not modify the information of a signal but mayadjust its current level, voltage level, and/or power level. Inferredcoupling, for example where one element is coupled to another element byinference, includes direct and indirect coupling between two elements inthe same manner as “coupled”.

While the present disclosure describes various embodiments, theseembodiments are to be understood as illustrative and do not limit theclaim scope. Many variations, modifications, additions and improvementsof the described embodiments are possible. For example, those havingordinary skill in the art will readily implement the steps necessary toprovide the structures and methods disclosed herein, and will understandthat the process parameters, materials, and dimensions are given by wayof example only. The parameters, materials, and dimensions can be variedto achieve the desired structure as well as modifications, which arewithin the scope of the claims. Variations and modifications of theembodiments disclosed herein may also be made while remaining within thescope of the following claims. For example, although the description hassome focus on the pacemaker, system, structures, and techniques canotherwise be applicable to other uses, for example multi-site pacing forprevention of tachycardias in the atria or ventricles. Phraseology andterminology employed herein are for the purpose of the description andshould not be regarded as limiting. With respect to the description,optimum dimensional relationships for the component parts are to includevariations in size, materials, shape, form, function and manner ofoperation, assembly and use that are deemed readily apparent and obviousto one of ordinary skill in the art and all equivalent relationships tothose illustrated in the drawings and described in the specification areintended to be encompassed by the present description. Therefore, theforegoing is considered as illustrative only of the principles ofstructure and operation. Numerous modifications and changes will readilyoccur to those of ordinary skill in the art whereby the scope is notlimited to the exact construction and operation shown and described, andaccordingly, all suitable modifications and equivalents may be included.

What is claimed is:
 1. A cardiac pacing system comprising: asubcutaneous implantable cardioverter-defibrillator (SICD) configuredfor implantation within a torso of a patient, the SICD comprising: ahousing; an electric circuit contained within the housing; and asubcutaneous lead having one or more electrodes coupled to the electriccircuit, the one or more electrodes being configured to sense cardiacsignals in one or more chambers of the patient's heart; and at least oneleadless pacemaker configured for implantation in at least one cardiacchamber of the patient and configured for leadless cardiac pacing, theat least one leadless pacemaker comprising: a housing; at least a firstelectrode formed integrally to the housing or coupled to the housing ofthe at least one leadless pacemaker at a maximum distance of 2centimeters; at least a second electrode formed integrally to thehousing or coupled to the housing of the at least one leadless pacemakerat a maximum distance of 2 centimeters; a pulse generator hermeticallycontained within the housing of the at least one leadless pacemaker andelectrically coupled to at least the first and second electrodes of theat least one leadless pacemaker, the pulse generator configured togenerate and deliver electrical pulses via at least the first and secondelectrodes of the leadless cardiac pacemaker to cause cardiaccontractions; circuitry contained within the housing of the at least oneleadless pacemaker, wherein the circuitry consumes no more than 64microwatts, the circuitry comprising a processor hermetically containedwithin the housing of the at least one leadless pacemaker andcommunicatively coupled to the pulse generator and at least the firstand second electrodes of the at least one leadless pacemaker, theprocessor configured to control electrical pulse delivery according toprogrammed instructions, wherein the SICD is configured to transmitprogramming instructions to the at least one leadless pacemaker; and aprimary power supply hermetically contained within the housing of the atleast one leadless pacemaker and coupled to the pulse generator, theprimary power supply configured to supply energy for operations andelectrical pulse generation as a source internal to the housing of theat least one leadless pacemaker.
 2. The cardiac pacing system of claim1, wherein: the at least one leadless pacemaker further comprises acapacitor coupled across a pair of the at least first and secondelectrodes of the leadless pacemaker and adapted for charging anddischarging wherein an electrical pulse is generated; the pulsegenerator is further configured to convey information to the SICD byconductive communication via the at least first and second electrodes ofthe leadless pacemaker used for delivering the electrical pulses tocause cardiac contractions by discharging the capacitor to form anelectrical pulse during a refractory period of the heart and encodingthe information to be conveyed on the electrical pulse; and the powersupply is further configured to supply power for said conductivecommunication.
 3. The cardiac pacing system of claim 2, wherein thecircuitry of the at least one leadless pacemaker further comprises: acharge pump circuit coupled to the capacitor and adapted for controllingcharging of the capacitor, wherein the processor is further configuredto control recharging of the capacitor, wherein recharging isdiscontinued when a power source terminal voltage falls below apredetermined value to ensure sufficient voltage for powering the atleast one leadless cardiac pacemaker.
 4. The cardiac pacing system ofclaim 1, wherein the primary power supply is configured to source nomore than 70 microwatts instantaneously.
 5. The cardiac pacing system ofclaim 1, wherein the electric circuit contained within the housing ofthe SICD is configured to at least one transmit and receive conductedcommunication using the one or more electrodes of the subcutaneous lead.6. The cardiac pacing system of claim 1, wherein the at least first andsecond electrodes of the at least one leadless pacemaker are configuredfor receiving a signal having a pulse duration; and wherein theprocessor of the at least one leadless pacemaker comprises a controllerconfigured to determine whether the signal is a triggering signal basedon the pulse duration of the signal.
 7. The cardiac pacing system ofclaim 1, wherein the circuitry of the at least one leadless pacemakercomprises a controller configured to: determine whether a triggeringsignal has arrived within a predetermined limit and, when the triggeringsignal has arrived within a predetermined limit, activate delivery of apacing pulse following a predetermined delay.